In a previous post I’ve been talking about the high number of UMTS and LTE networks I’ve been observing in Bangalore and that I had did not see the kind of network sharing in my traces that I was expecting. I didn’t go into the details of the different kinds of network sharing in this post so I’ll follow up with the details here.
Basically a base station can be shared two ways:
MOCN (Multi-Operator Core Network)
In this sharing approach the base station is shared by broadcasting several Mobile Country Codes (MCC) and Mobile Network Codes (MNC) in the System Information of a radio channel. In case of UMTS this is done in the Master Information Block (MIB) while in LTE this is done in the System Information Block (SIB) 1. Here’s an example of how this looks like for an LTE base station I’ve seen in Kansas City shared by T-Mobile and Metro PCS:
SystemInformationBlockType1: [...] CellAccessRelatedInfo: PLMN Identity List PLMN-Identity mcc 310 mnc 260 cellReservedForOperatorUse notReserved PLMN-Identity mcc 311 mnc 660 cellReservedForOperatorUse notReserved trackingAreaCode '10100101 00000001' cellIdentity '00000100 11101011 10110000 0010' cellBarred notBarred [...]
Note that the Tracking Area Code and the Cell Identity are the same for both networks!
How The Mobile Informs The Base Station To Which Operator It Wants To Talk To
When the mobile connects to the network it includes information to which core network it would like to talk to. In UMTS this is done in the Initial Direct Transfer message in which the first NAS message (Location Update Request) is embedded. IMPORTANT: The selected PLMN identity is part of the Initial Direct Transfer message and NOT the embedded Location Update message! In the Location Update message there is a PLMN id but this is the last network + last cell an NOT the selected network. For details see 3GPP 25.331, 10.2.16c (Initial Direct Transfer). Note: The selected PLMN-id is also NOT part of the RRC Connection Request. The PLMN ID included in this message is also the previous cell + PLMN!!!
MORAN (Mobile Operator Radio Access Network)
If each of the network operators sharing a base station uses his own spectrum (i.e. his own radio channel) the operators share the backhaul link from the base station to the core network and the digital module of the base station but have separate RF paths and radio units. A simple mobile device trace analysis can’t reveal if the two signals come from the same base station or not. One would have to compare the network configuration parameters in the system information messages carefully but even here there might be differences.